There is really no slant range in GPS Navagation. It is Triangulation &
Timing.
You need at least 3 satellites in view to get an accurate 3D position.
Distance is
determined by accurate timing of how long it takes an encoded signal to
reach
the GPS reciever. Thus with known distances from 3 satellites you can
calculate or triangulate (within the tolerance) the exact position of the
GPS reciever. This position
includes altitude, although altitude in my experience seems to be less
accurate. But for all I know the GPS altitude may be more accurate than
altitude read from my altimeter, which is
corrected for non standard pressure. Hope this short explanation helps

Mark



"David W" wrote in message
ups.com...
What does 'optimised' for GPS mean?

I'm not sure. Somebody with whom I am having a 'debate' has asserted
that there is an altitude-dependent error component (if I may call it
that) on positions determined by GPS (and I presume that this alleged
error component affects the horizontal component of a 3D position, as
well as vertical (altitude) component). In his own words:

"GPS is optimised for sea level, Blanchefort [a mountaintop ruined
castle] is 467 metres above sea level, couple this with a slant range
to a satellite of several thousand miles and the curvature of the earth
and you have error. At least up to 100 metres..."

I personally can find no evidence which supports his claim that a) GPS
is optimised for sea level, or b) GPS coordinates obtained at a few
hundred metres above (mean)
sea level are affected by his alleged altitude-induced error.

I replied (verbatim quote - please excuse the sarcastic tone!):

"This is really bad news. Modern aircraft - many of which rely heavily
on GPS for navigation - are in big trouble then, aren't they? I mean,
if the error at just 467 metres above MSL is ''at least up to 100
metres'', then surely it must be several kilometres by the time we get
up to altitudes like 30,000 ft., right?"

His reply (with some non-essentials removed):

"Aircraft ... are using a map optimised to the GPS system and this is
the key point which seems to be passing you by."


What's behind your questions; what are you trying to get at? If you
posted
that, you might get more suitable answers.

I hope that that is sufficiently answered above. I didn't want to
burden this group with extensive background information and endless
quotes from this slightly silly debate!


Regards,

David,
England.